1. Field of the Invention
The invention relates to the field of direct current power supplies having starting devices for powering loads such as gas discharge devices including fluorescent lamps, halogen lamps, arc lamps and the like. In particular, the invention provides an electronic ballast circuit having two constant power regulators and a voltage multiplying starting circuit having a current output that varies with load resistance and is loaded to the point that starter current is turned substantially off after the load has started.
2. Prior Art
Gas discharge devices such as fluorescent and halogen lamps, typically having gases enclosed in a glass envelope, emit light when gases are ionized and during conduction of current through the ionized gases, their electrons change energy state. The typical lamp comprises an elongated tube with electrodes at opposite ends, although other configurations also are possible. In order to start the lamp, the gas must be ionized. This can be accomplished by keeping the gas heated, or by applying a high voltage starting signal. Once the gas is ionized, the resistance between the electrodes (i.e., through the gas) is reduced substantially due to ionization. In the steady state mode of operation, the voltage needed to emit light is lower than the starting voltage. So-called "warm-start" or "rapid-start" ballasts for such lamps dissipate power when the lamp is off, to maintain its readiness to start. "Instant-start" ballasts apply a high voltage spike to ionize the gas and start the lamp.
Electronic ballasts for fluorescent lamps can offer better efficiency, faster start times, and higher power efficiency compared with traditional inductor ballasts. A major drawback of electronic ballasts is their cost as compared to inductor ballasts. The costs of parts and manufacturing are typically much higher for electronic ballasts because many more parts and connections are needed.
Electronic ballasts can be distinguished between AC ballasts and DC ballasts, depending on the fluorescent lamp's working mode. In an AC ballast, a high frequency AC voltage is applied to the electrodes of a fluorescent bulb, for example at 10 to 50 KHz, and typically at 25 to 35 KHz. As shown, for example, in U.S. Pat. No. 5,021,716--Lesea, an AC signal is generated by an oscillator and a voltage step-up transformer. A DC ballast is also possible wherein a DC voltage is applied across the electrodes. Most commercial electronic ballasts are AC types, because it is readily possible to start and to control the lamp output level at a stable working point.
The AC output is coupled to the electrodes via a power coupling high frequency transformer, and control of the electronic ballast typically is based on a current feedback signal, requiring a second transformer for sensing. Such high frequency transformers are also expensive. In order to obtain feedback of the average signal level to the control, AC ballasts generally need high accuracy components. Special tuning and setup may be necessary during manufacturing, which increases the manufacturing cost. It would be advantageous if the cost of an electronic ballast could be reduced without sacrificing efficiency and good control characteristics, perhaps including dimming control.
Another drawback of AC ballasts is the emission of electromagnetic radiation at the basic working frequencies (10 to 50 KHz), and their harmonics. The AC current in the lamp at such frequencies causes an antenna effect, and the power couplings through the transformer and along connecting conductors also radiate at such frequencies. This radiation is a kind of electromagnetic pollution. Whether or not electromagnetic radiation is harmful to humans, such radiation produces adverse effects in instrumentation in which signals are induced. For example, in environments having electronic test equipment, such as in hospitals, electronic ballasts are generally not useful due to the induction of high frequency signals in test equipment such as electrocardiograph machines and the like.
A DC ballast produces little electromagnetic radiation and need not include expensive high frequency transformers. However, there can be problems with dependably starting the lamp. It can be difficult to control the current level to a stable working point. A cataphoresis or electrophoresis effect is observed wherein the light output near the anode decreases compared to the light output near the cathode, because the direct current causes migration of positively charged mercury ions toward the cathode. These problems are not encountered in AC ballasts.
U.S. Pat. Nos. 4,777,409--Tracy et al. and 4,983,888--Akutsu et al. disclose DC type ballasts for a small (4 inch or 5 cm) exit sign bulb, and for a U-shaped combined fluorescent and incandescent bulb arrangement, respectively. In each case the DC ballasts include a bridge rectifier and chopper means associated with a series regulating transistor, for generating a pulsed DC signal that is filtered for applying a smoothed and rectified signal to the gas discharge tube. In Tracy, the lamp is started by applying the rectifier output to heat the electrodes resistively. The electrodes each have two terminals and initially are coupled to the regulator output by a bimetallic starter in series with the electrodes. As the electrodes are heated, the starter heats up and opens. Operating current is then limited by feedback to the chopper via a current sensing resistor in series with the bulb.
In Akutsu et al., a separate starter device can be coupled to the discharge tube as in Tracy, or according to additional alternatives, a capacitor in parallel with the electrodes can allow a starting current to heat the electrodes until it is charged, or a switching device can initially apply a heating current to the electrodes. An incandescent lamp is provided in series with the fluorescent lamp and presumably helps to heat the gases in the tube. Akutsu combats electrophoresis effects by using a special tube. A mercury amalgam insert at the anode diffuses additional mercury from the insert into the tube, which is said to provide a fresh supply of electrons that counteracts the tendency of the positively charged ions to migrate to the cathode.
It would be advantageous to provide a DC electronic ballast that does not need special amalgams, and yet which overcomes problems with electrophoresis. It would also be advantageous to provide a starter for a DC electronic ballast that does not resistively heat the electrodes, which tends to wear them and contribute to premature failure of the lamp. It would also be advantageous to provide a DC electronic ballast that can be controlled for dimming and has a minimum of components.